Please refer to FIG. 1. A conventional projection display 10 mainly includes a case 11, and a lamp 12, an optical engine 13, a control unit 14 and a set of fan 151 and fan 152 are disposed inside the case 11. Because the lamp 12, the optical engine 13, the control unit 14 generate heat, the set of fan 151 and fan 152 is needed for cooling so as to maintain the normal operation of the projection display 10.
However, the environment of the projection display 10 changes according to a user's need, such as: a high elevation above sea level or a low elevation above sea level. The heat conduction of the projection display 10 is according to the following formula:Q=ρ*Cp*V*□T wherein, Q is the total heat, ρ is the density of the air, Cp is the specific heat of the air, V is the airflow rate of the fan and □T is the temperature difference of the air flowing out of the projection display after being heated and the air flowing in the projection display. The number of ρ decreases with the increasing of the elevation above sea level, thus, air of same volume can take away more heat on a high elevation above sea level than a low elevation above sea level. Consequently, the set of fan 151 and fan 151 that provides constant airflow cannot take away the heat inside the projection display completely, and then heat is accumulated in the projection display device 10 when at a high elevation above sea level, resulting the temperature of each component rises and each optical component in the projection display is damaged, and thereafter the lifetime of each component in the projection display is shortened. In addition, if the influence of the elevation above sea level is considered in the beginning of design, the noise will become louder and a user need to put up with a noisy operation environment even at a normally low elevation above sea level.
Hence, it is to be researched that how to make the projection display 10 have the best cooling efficiency in different operation environments.